27 March 2011

The double-edged sword of salting roads in winter

The problem of road salt working its way into local waters is nicely explained in a StarTribune article:

This winter, the Pollution Control Agency (PCA) started a four-year project to figure out which Twin Cities' lakes hold too much chloride, a primary ingredient in salt, and what it will take to keep urban waters healthy...

Shingle Creek, which flows under highways and behind shopping malls in the western suburbs, is a case in point. It's in one of the few watersheds where the salt load has been calculated. In theory, to get it back to healthy levels, the nine communities along its banks would have to cut salt use by a whopping 71 percent... "When your tradeoff is public safety, it's very difficult."

Road salt use has been rising steadily for more than 20 years. Now, in an average winter, some 350,000 tons of salt are dropped on roads, sidewalks and parking lots in the Twin Cities metropolitan area... Eric Novotny, one of the researchers on the university chloride study, found that 78 percent of the salt applied to roads stays in the water. Unlike some other pollutants, it does not flow to the Mississippi and down to the Gulf of Mexico. Instead, the denser, salty water sinks to the bottom of lakes and into groundwater, accumulating year after year...

Brownie Lake, a small, deep lake in Minneapolis near Cedar Lake Parkway and Interstate 394, contains so much chloride that its top and bottom layers no longer mix, which is vital to a healthy lake. In part, it doesn't turn over because the lake is so deep relative to its surface area, and pollutants of all kinds flow down its steep banks. But salt has been collecting in its depths for years. Two meters below the surface, the water in Brownie contains virtually no oxygen, said Rachael Crabb, a water quality expert for the Minneapolis Parks Department, indicating there's not much life at the bottom. "Whatever chloride has come into Brownie from 394 is still there, and it's going to say there," she said...

In recent years, the city bought new, high-tech salt trucks, and began making its own brine to apply to roads before a snowstorm. It also uses GPS units to precisely regulate exactly how much salt the trucks put down in any location. Since 2007, the amount of salt used per mile per snowstorm has been cut in half, Albrecht said. It took awhile to educate Prior Lake citizens about the project, and to convince them that roads were still safe, even if they were not cleared down to bare pavement...

More at the link. The layering of lake waters, with brine in the depths, is reminiscent of the situation in the Black Sea, where there is a pycnocline.

Addendum: New word for the day: A lake whose layers do not mix is called a meromictic lake, in contrast to a normal holomictic lake.

Image source (showing thermal layering, but the principles are also applicable re salinity)

On my recent visit to Canada I was flabbergasted about the amount of salt they use.

Here in Finland we are going the other way, road salting has dropped to a small fraction what is used to be. Only the biggest main roads are salted, and even them very lightly. Most of the winter the only place you can see asfalt is on the freeway. Sidewalks are never salted, instead we only use gravel for some traction.

Winter tires are mandatory, and most people use studded ones. As a bonus, people learn to drive on slippery roads :)

One of the alternatives I have heard for de-icing roads in winter was sand, which wouldn't be too detrimental I think, considering it's natural. From what I've heard and read, the reason why cities and states use salt is due to cost: apparently it's cheaper than sand, even though it's more detrimental to cars and the environment.

On a side note, I remember they used to salt a bridge at a school I attended, due to the fact that it would freeze and ice up even in warmer months.

Salting the roads before a winter storm is downright dangerous. It produces so much slush, which is the worst possible thing to drive on next to wet clay mud. I think salting before a snow or even during is massively irresponsible. Most of the salt applied in bad weather just gets wasted anyhow. Secondly, road salt often has heavy metals in it, and has been linked to higher cancer rates.http://www.elements.nb.ca/theme/transportation/salt/salt.htm

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